Lignocellulosic materials are sources for the generation of a variety of products. Some of the products retain significant structural components of the lignocellulose such as mechanical pulp fibers from wood chips. Other compounds such as sugars derived from the carbohydrate in lignocellulose are made into products by fermentation or chemical conversion. The lignocellulose can be made into products that represent a continuum of structured to molecular products. The continuum of products is generated by a variety of physical, chemical, biological and thermal processes.
Medium-density fiberboard (MDF) is an engineered wood product formed by lignocellulosic fibers glued under heat, pressure and a small amount of resin. In manufacturing MDF from wood, the wood is first reduced to an intermediate stage in which the fibers in the wood are separated from their natural environment and transformed into a pulp like suspension. One of the components of wood is lignocellulose. The most abundant component of lignocellulose is the cellulose polymers. These are the most desired polymers in the final product. The second most abundant polymer is lignin. Lignin is useful as a material which allows fibers to adhere in MDF.
The third major component of lignocellulose is the hemicellulose. Hemicelluloses are polymers of sugars that are more heterogeneous than cellulose. The hemicelluloses are comprised of oligomeric sugars derived from arabinose, galactose, xylose and mannose in addition to glucose. The hemicellulose and the lignin are intermixed with the cellulose in lignocellulose and serve to protect the cellulose from damage by organisms, enzymes or chemicals. Removal of the hemicellulose and lignin is often a portion of lignocellulose processing.
Pulp suitable for MDF production may be produced from various types of lignocellulose using any one of several techniques. The simplest of these techniques is the refiner mechanical pulping (RMP) method in which a mechanical milling operation grinds or abrades wood in water until a desired state of freeness (an arbitrary measure of water drainage) is achieved between its fibers. The RMP method is high yield, typically converting approximately 95% of the dry weight of the wood into pulp. The RMP method leaves most of the lignin and hemicellulose in the pulp.
Other mechanical pulping methodologies include thermo-mechanical pulping (TMP), chemical treatment with thermo-mechanical pulping (CTMP), and chemi-mechanical pulping (CMP). Alternatively there are also chemical pulping methods, wherein a chemical/water solution is generally used to dissolve the lignin and hemicellulose to promote the separation of the fibers.
In thermo-mechanical processes (e.g. TMP and CTMP), high temperatures are used to help separate the fibers during refining. These processes generally require the refining to be carried out in one or more steps. The first step is usually a pressurized step with refining being performed at temperatures above 100° C. and immediately below or at the softening temperature of lignin. During this step, the pulp is typically mechanically processed using the TMP method. In subsequent steps, the pressure and temperature is usually modulated to achieve the desired state of freeness between the fibers.
Relatively high total electric energy amounts or high quantities of input lignocellulose are required to produce pulps using the above mentioned mechanical pulping techniques. In particular, high energy inputs are generally required to obtain fiber separation in woods rich in lignin as such woods typically call for extended refining periods and higher refining temperatures or pressures. Recent studies have also suggested that even thermal or chemical softening treatments of such woods do not guarantee a lower total energy consumption. This is because unprocessed fibers which are only mildly separated by the thermal or chemical treatments are difficult to fibrillate during the mechanical refining process.
Fibrillation is necessary to increase the flexibility of the fibers and bring about the fine material characteristics of quality processed pulp. In fact, it has been suggested that a decrease in the energy consumption from an established level in various TMP and CTMP processes has been associated with the deterioration of certain pulp properties, including a reduction in the long fiber content of the pulp (See U.S. Pat. No. 5,853,534, which is incorporated by reference here in its entirety). As a result, high energy consumption in TMP and CTMP processes has been generally necessary in today's pulping practices.
Along with the problems of the high energy cost of pulping, MDF has the problem of generally having low moisture tolerance. MDF, therefore, often swells when contacted with water and is therefore of more limited use in outdoor settings and in applications that encounter moisture such as furniture surfaces. Absorption of moisture also often causes significant deterioration in the mechanical integrity of the MDF.
In regards to reducing the energy costs of pulping, applicants invented a method for producing pulp from fibrous lignocellulose material using a treatment step which exposes the material to oxalic acid, or oxalic acid and sodium bisulfite, prior to pulping. Applicants discovered that pulping the resulting product actually required less energy input and provided a pulp with enhanced physical properties as compared to untreated fibrous lignocellulose material (see, U.S. Patent Publication No. 20030041985). From this applicants determined that pretreatment with oxalic acid or oxalic acid derivatives reduced energy requirements in mechanical refining for making paper. This technology is different from that which is described hereinbelow for MDF, as the properties of fibers relevant to making paper sheets are specific to paper products and are not related to the properties that fibers will exhibit when formed into MDF. In particular, the methods described below produce MDF that has, in-part, improved water repellency, a resulting characteristic that is not observable in the papermaking process.
Accordingly, an improved method is needed for producing pulp which is energy efficient and is able to produce MDF having improved water repellency properties.